Voltammetry is a group of techniques involving very accurate measurements of current flow as a function of potential (voltage) over a period of time. Usually the potential of the working electrode is controlled to precisely known values (which may vary in a controlled manner as a function of time) with respect to a reference electrode placed in a solution to be analyzed. Any electroactive substance in the solution will transfer electrons to (anodic oxidation), or accept electrons from (cathodic reduction), the external circuit at the active surface of the working electrode whenever the potential is in a characteristic range. The magnitude of the current is proportional to the concentration of the substance in solution and the characteristic potential depends on the identity of the substance; thus voltammetry can be used for both quantitative and qualitative electrochemical analysis.
Voltammetry is often carried out with a three-electrode configuration in an electrochemical cell containing electrolyte solutions, which may be purely aqueous, nonaqueous or mixtures of water and a solvent. A potentiostat is employed to accurately control the potential of a working electrode with respect to a reference electrode by forcing the necessary current through an auxiliary electrode. This current also passes through the working electrode and is measured using any known current to voltage transducer. Sometimes the control potential, also called the applied potential, is varied over time according to a predetermined program, for example to analyze multiple species in the sample.
Liquid mercury as an electrode material in electrochemical research is widely accepted because of its good physical and electrical characteristics. It has a wide liquid temperature range (-38.9.degree. TO 356.9.degree. C.) and electrodes of various shapes can be easily prepared. In contrast to solid electrode materials, the active surface of such electrodes is highly uniform and easily reproducible if the mercury is clean. Most importantly, mercury also has a very high overvoltage for hydrogen evolution relative to other metallic and carbon electrodes so that electrochemical reactions that require more negative potentials can be carried out without as much background interference.
Several types of apparatus utilizing liquid mercury electrodes have been developed since their invention circa 1922 to help simplify or improve electrochemical investigations. Among those which are well known to those skilled in this art are: dropping mercury electrodes (DME), hanging mercury drop electrodes (HMDE), static mercury drop electrodes (SMDE), streaming mercury electrodes (SME), mercury film electrodes (MFE), and, more recently, controlled growth mercury electrodes (CGME). See, for example, U.S. Pat. No. 4,548,679 (Guidelli et al) which discloses apparatus for the automatic control of the growth or size of a hanging mercury drop; U.S. Pat. No. 4,661,210 (Tenygl) which discloses methods and apparatus for electrochemical analysis of solutions by voltammetry using a pulsating liquid mercury electrode and solution within a small volume capillary tube; U.S. Pat. No. 4,846,955 (Osteryoung et at) which also relates to the control of growth or size of a mercury drop electrode; U.S. Pat. No. 5,131,999 (Gunasingham) discloses a flow cell using a renewable mercury electrode; U.S. Pat. No. 5,292,423 (Wang) which relates to methods and apparatus for trace metal testing using mercury coated, screen-printed flat film electrodes; U.S. Pat. No. 5,326,451 (Ekechukwu) which discloses a liquid dropping electrode not made of mercury for use in non-polar solutions; and U.S. Pat. No. 5,378,343 (Kounaves et al) also disclosing mercury coated flat film electrodes.
However, these prior art devices have a number of disadvantages when used to determine trace amounts of analytes, drugs or poisons, in samples of complex biological fluids, such as blood, urine, tissue homogenates, or in environmental samples.
One application of increasing importance is the determination of heavy metals (e.g. lead) in biological samples (e.g. blood). Currently, the most widely used analytical techniques are atomic absorption and inductively coupled plasma spectroscopy but, due to the advanced nature of these methods, highly trained personnel and expensive equipment is required. Scientists would like to use anodic stripping voltammetry for this purpose wherein the metal ions present in the sample would be electroplated onto a mercury electrode and subsequently removed (stripped) electrochemically. The current recorded during stripping is proportional to the concentration of the metal of interest. This method would be highly sensitive to many metal ions and could be operated at much lower cost than the commonly used methods. Potentiometric Stripping Analysis is another variation on this same theme which would be useful with improved electrodes.
Although Mercury Drop Electrodes provide a means for continuously renewing the electrode surface by releasing a used drop and forming a new one, it is still directly in contact with the sample media and hence electrode contamination or fouling from common biological components in the sample is a significant problem. Also, all mercury drop techniques introduce used mercury drops into the sample container during the analysis which contaminates the sample and makes recovery of the mercury, for proper disposal as a health hazard, or further use of the sample, which may be quite precious, difficult at best. Apart from this, currently available designs of mercury drop electrodes do not allow use of very small, microliter, volume samples which are usually the amounts available in biological research.
Routine use of a mercury electrode could be substantially increased if one could be incorporated into a flow cell arrangement, i.e. to analyze a flowing stream of analyte solution, such as in liquid chromatographic detection of reducible organic compounds. However, the stability of a mercury drop in a flow cell is greatly reduced and consequently the background electrical noise in the analytical signal is so high that the accurately processing of the data is jeopardized. The detection limit achievable in a particular analysis under these conditions is unavoidably high and hence the quality of analytical data becomes unsatisfactory.
Mercury Film Electrodes can, on the other hand, be easily incorporated into a flow cell but their stability, and therefore data reproducibility, is dependent on the adhesion of the mercury film to the substrate material. In addition, MFEs on different substrates perform differently based on the substrate surface characteristics, thereby questioning the integrity of the analysis. Metallic substrates provide better adhesion than carbon surfaces but the solubility of the solid surface in the mercury discourages long term use. For example, mercury plated onto a gold surface will slowly dissolve the gold so that after a time, the active electrode surface becomes a gold/mercury amalgam with different electrical characteristics. The useful lifetime of a film electrode depends on the type of base metal, the amount of mercury plated and its operating conditions. In the case of a carbon base surface, there is no alloying interaction that changes the nature of the mercury but the porosity of the surface affects some applications and the cleaning of the surface or recovery of metals deposited on the electrode during anodic stripping voltammetry can be difficult if not impossible. Further, mercury films, especially on carbon substrates, which are subject to flowing streams of analyte can change surface area continuously, due to loss of mercury, thereby degrading the confidence and reproducibility of the analytical data.
Mercury Film Electrodes have been coated with various polymeric materials in order to try to circumvent the fouling problems by partially filtering the unwanted electroactive and surface active species but the reproduction of a particular coating thickness, which is necessary to maintain a constant performance level from time to time, is inherently very difficult.
It is therefore a general object of the present invention to provide a new and improved method and apparatus for forming a liquid mercury electrode and in particular an electrode having a small, renewable active analyzing surface, which is protected from fouling, usable in stationary or flowing analytical solutions and without contaminating such solutions with used mercury.